Jennings is a member of a group known as "The Trust" which is comprised of former rogue NID agents who believe that their mission to protect Earth at any cost gives them the right to operate illegally. He was introduced in the episode, 8.07 "Affinity".
Jennings blackmailed Daniel Jackson into doing a translation for "The Trust" in exchange for clearing Teal'c from kidnapping and murder charges they had framed him for, and for saving the life of one of Teal'c's neighbors, Krista James (7.07 "Affinity).
Daniel and Carter later encountered Jennings after he, along with Hoskins and Brooks, gained access to Osiris' cloaked ship left in orbit from the episode, 7.15 "Chimera". As it turns out, the translation he coerced Daniel into making gave The Trust access to the control systems of the ship and enabled them to steal the Stargate out of the SGC and launch small rockets carrying symbiote poison to planets known to be under Ba'al's rule through it. Along with Teal'c, Daniel and Carter were able to get the Stargate back and transport off of the ship, but Brooks was able to take the ship through a hyperspace window and escape. Jennings' group leader, Hoskins, however, was killed by Teal'c with his staff weapon, while Jennings had been kicked to unconsciousness by Carter. (8.10 "Endgame")
Jennings and Brooks released the symbiote poison on two more planets before they were captured by a Goa'uld and implanted with symbiotes. Brooks' status after this point is not known, but Jennings returned to Earth and began implanting symbiotes in other members of The Trust, namely the wealthy businessmen who financed the group's activities. Jennings also had a hand in implanting former Vice President Robert Kinsey with a symbiote. After doing so, they all beamed up to their al'kesh and began coordinating their effort to cause a nuclear war between the superpowers of Earth while they took control of the weapon in the Antarctic Outpost. Kinsey was sent to Russia to stir up suspicion between the U.S. and Russian governments, but he was subsequently taken by Daniel Jackson to the Prometheus. Jennings attempted to destroy the Prometheus while they were in orbit and his attack gave Kinsey the opportunity to escape to the al'kesh. Kinsey shot and killed Jennings when he figured that Jennings was trying to destroy the Prometheus because he was onboard it. Kinsey took the hand device (ribbon device) which controlled the transporter technology, so it is possible that he beamed off of the ship before the Prometheus destroyed it. (8.14 "Full Alert")
--DeeKayP 14:58, 25 Aug 2004 (PDT)